Cooling system for fluid to be cooled

ABSTRACT

A cooling system includes a compressor configured to pressurize carbon dioxide to form pressurized carbon dioxide, a mixer configured to generate mixed refrigerant in which the pressurized carbon dioxide and solvent in a liquid state, a depressurization apparatus provided downstream from the mixer and configured to depressurize the mixed refrigerant, a separator configured to separate carbon dioxide in a gas state from the mixed refrigerant, a heat exchanger configured to exchange heat between the mixed refrigerant cooled through depressurization and a fluid to be cooled, and a second heat exchanger configured to cool the carbon dioxide or the mixed refrigerant using vaporized carbon dioxide or the mixed refrigerant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is divisional of U.S. Application Ser. No. 16/574,533,filed Sep. 18, 2019, which is a continuation of InternationalApplication No. PCT/JP2018/035620, filed Sep. 26, 2018, which claimspriority to Japanese Patent Application No. 2018-070008, filed Mar. 30,2018, the contents of each of which are incorporated herein by referencein their entireties.

TECHNICAL FIELD

This disclosure relates to a cooling system.

BACKGROUND

Conventionally, flammable fluids that are gases at normal temperaturesand pressures, such as propane, have been employed as refrigerants inlarge-sized cooling apparatus. However, flammable fluids that are gasesat normal temperatures and pressures require rigorous countermeasuresagainst leakage or the like, and thus cannot be easily handled. For thisreason, in recent times, the use of non-flammable fluids that are gasesat normal temperatures and pressures as refrigerants are beingconsidered. For example, Patent Literature 1 discloses a method usingliquefied carbon dioxide as a refrigerant.

DOCUMENT OF RELATED ART Patent Document

[Patent Document 1] Japanese Unexamined Patent Application, FirstPublication No. 2007-225142

SUMMARY Technical Problem

However, some carbon dioxide becomes solid (dry ice) when the carbondioxide is at a temperature of a triple point (−56.6° C.) or less. Forthis reason, when the temperature of the carbon dioxide is decreased to−56.6° C. or less in a cooling apparatus, the piping may be clogged dueto dry ice forming inside, and feeding carbon dioxide into the coolingcycle becomes difficult. Furthermore, formation of dry ice in anapparatus caused by an operation mistake or the like may interfere withoperation of the apparatus. In addition, increased energy efficiency incooling systems using carbon dioxide is required.

In consideration of the above-mentioned problems, this disclosure aimsto increase energy efficiency using carbon dioxide as a refrigerant,whilst further facilitating operation and improving reliability ofapparatuses.

Solution to Problem

A cooling system of the first aspect of this disclosure includes, as afirst means, a compressor configured to pressurize carbon dioxide toform pressurized carbon dioxide; a mixer configured to generate a mixedrefrigerant in which the pressurized carbon dioxide and a solvent in aliquid state are mixed; a depressurization apparatus provided downstreamfrom the mixer and configured to depressurize the mixed refrigerant; aseparator configured to separate carbon dioxide in a gas state from themixed refrigerant; a heat exchanger configured to exchange heat betweenthe mixed refrigerant cooled through depressurization and the fluid tobe cooled; and a second heat exchanger configured to cool thepressurized carbon dioxide or the mixed refrigerant using a vaporizedcarbon dioxide or the mixed refrigerant.

In the cooling system of the first embodiment of this disclosure, thedepressurization apparatus may include a power recovery turbine, and apower recovery apparatus configured to collect kinetic energy of themixed refrigerant from the power recovery turbine.

In the cooling system of the first embodiment of this disclosure, thesecond heat exchanger may be provided upstream from the depressurizationapparatus and configured to cool the solvent using the vaporized carbondioxide or the depressurized mixed refrigerant.

In the cooling system of the first embodiment of this disclosure, thesecond heat exchanger may be provided downstream from thedepressurization apparatus and configured to cool the mixed refrigerantbranched off upstream from the depressurization apparatus using themixed refrigerant depressurized in the depressurization apparatus.

In the cooling system of the first embodiment of this disclosure, thesecond heat exchanger may be integrated with the heat exchanger.

In the cooling system of the first embodiment of this disclosure, themixed refrigerant may be configured to be depressurized in a pluralityof steps, and a plurality of heat exchangers and a plurality of secondheat exchangers may be provided and connected to each other in series.

The cooling system of the first embodiment of this disclosure mayinclude a compressor configured to pressurize carbon dioxide to formpressurized carbon dioxide; a mixer configured to generate a mixedrefrigerant in which the pressurized carbon dioxide and a solvent in aliquid state are mixed; a depressurization apparatus provided upstreamfrom the mixer and configured to depressurize the pressurized carbondioxide; a separator configured to separate carbon dioxide in a gasstate from the mixed refrigerant; a heat exchanger configured toexchange heat between the mixed refrigerant cooled throughdepressurization and a fluid to be cooled; and a second heat exchangerconfigured to cool the pressurized carbon dioxide or the mixedrefrigerant using vaporized carbon dioxide or the mixed refrigerant.

Effects

According to this disclosure, when a second heat exchanger is provided,pressurized carbon dioxide, a mixed refrigerant or a solvent can becooled by low temperature carbon dioxide or the mixed refrigerantvaporized through depressurization. Accordingly, energy efficiency canbe improved by decreasing a temperature of the pressurized carbondioxide and the solvent and decompressing them. Further, since the mixedrefrigerant is used, the probability of piping clogging due togeneration of dry ice in the pipe can be reduced, and thus thereliability of an apparatus can be improved while operation thereof isfacilitated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a first embodiment of a coolingsystem according to an embodiment of this disclosure.

FIG. 2 is a schematic view showing a second embodiment of the coolingsystem according to the embodiment of this disclosure.

FIG. 3 is a partial schematic view showing a heat exchanger in a variantof the cooling system according to the embodiment of this disclosure.

FIG. 4 is a partial schematic view showing a heat exchanger in a variantof the cooling system according to the embodiment of this disclosure.

FIG. 5 is a partial schematic view showing a heat exchanger in a variantof the cooling system according to the embodiment of this disclosure.

FIG. 6 is a schematic view showing a variant of the cooling systemaccording to the embodiment of this disclosure.

FIG. 7 is a schematic view showing a third embodiment of the coolingsystem according to the embodiment of this disclosure.

FIG. 8 is a schematic view showing a variant of the cooling systemaccording to the embodiment of this disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of a cooling system according to thisdisclosure will be described with reference to the accompanyingdrawings.

First Embodiment

A cooling system 1A according to this embodiment is a system configuredto pre-cool natural gas in a normal temperature state, and as shown inFIG. 1 , includes a compressor 2, a cooler 3, a heat exchange apparatus5, a mixer 6, a depressurization apparatus 7, a mixed refrigerant-carbondioxide separator 8, a solvent-carbon dioxide separator 9 and a solventpumping apparatus 10. In addition, a natural gas cooling system (notshown) using nitrogen as a refrigerant is provided downstream from thecooling system 1A.

The compressor 2 includes a motor 2 a. The compressor 2 is an apparatusconfigured to pressurize carbon dioxide of about 0.5 MPaG to about 10MPaG to form pressurized carbon dioxide.

The cooler 3 is an apparatus provided downstream from the compressor 2and configured to cool the carbon dioxide having a high temperature bybeing pressurized by the compressor 2 using cooling water or the like.When the carbon dioxide passes through the cooler 3, the temperature ofthe carbon dioxide is about 40° C.

The heat exchange apparatus 5 is a multi-stream heat exchanger, forexample, a plate fin type heat exchanger, and includes a natural gasflow path 5 a, a mixed refrigerant flow path 5 c, a vaporized carbondioxide flow path 5 d, a solvent flow path 5 e and a pressurized carbondioxide flow path 5 f.

Natural gas in a normal temperature (about 25° C.) state is supplied tothe natural gas flow path 5 a. The supplied natural gas is cooled toabout −50° C. when the natural gas passes through the natural gas flowpath 5 a.

The mixed refrigerant (about −55° C.) depressurized in thedepressurization apparatus 7, cooled mainly by vaporization of thecarbon dioxide, and from which carbon dioxide in a gas state isseparated by the mixed refrigerant-carbon dioxide separator 8, issupplied into the mixed refrigerant flow path 5 c from a directionopposite to the natural gas flow path 5 a.

The carbon dioxide (about −55° C.) in a gas state separated in the mixedrefrigerant-carbon dioxide separator 8 is supplied into the vaporizedcarbon dioxide flow path 5 d in the same direction as the mixedrefrigerant flow path 5 c.

A solvent (a liquid such as methanol, ethanol, acetone, or the like) ofabout 35° C. is supplied into the solvent flow path 5 e in the samedirection as the natural gas. The supplied solvent passes through thesolvent flow path 5 e and then is cooled to about −45° C.

The pressurized carbon dioxide of about 40° C. and about 10 MPaG passingthrough the cooler 3 is supplied into the pressurized carbon dioxideflow path 5 f in the same direction as the natural gas. The pressurizedcarbon dioxide passes through the pressurized carbon dioxide flow path 5f and then is cooled to about −45° C.

In the above-mentioned heat exchange apparatus 5, a configurationincluding the natural gas flow path 5 a and the mixed refrigerant flowpath 5 c corresponds to a heat exchanger according to the disclosure,and a configuration including the vaporized carbon dioxide flow path 5d, the solvent flow path 5 e and the pressurized carbon dioxide flowpath 5 f corresponds to a second heat exchanger according to thedisclosure. That is, in this embodiment, flow paths of the second heatexchanger constitute a multi-stream heat exchange apparatus integratedwith the heat exchanger.

The mixer 6 is connected to the solvent flow path 5 e and thepressurized carbon dioxide flow path 5 f and has a stirring bar or thelike (not shown). The mixer 6 is an apparatus configured to mix thesolvent and the pressurized carbon dioxide and generate the mixedrefrigerant.

The depressurization apparatus 7 is an apparatus including a valveprovided on a flow path of a mixed refrigerant flowing from the mixer 6to the mixed refrigerant-carbon dioxide separator and configured todepressurize the mixed refrigerant from about 10 MPaG to about 0.5 MPaGto obtain a low temperature.

The mixed refrigerant-carbon dioxide separator 8 is an apparatusprovided downstream from the depressurization apparatus 7 and anapparatus configured to separate the mixed refrigerant and gaseouscarbon dioxide cooled to about −55° C. A mixed refrigerant feedingapparatus 8 a is provided below the mixed refrigerant-carbon dioxideseparator 8, and the mixed refrigerant flows to the mixed refrigerantflow path 5 c of the heat exchange apparatus 5 by the mixed refrigerantfeeding apparatus 8 a.

The solvent-carbon dioxide separator 9 is a tank provided downstreamfrom the mixed refrigerant flow path 5 c and the vaporized carbondioxide flow path 5 d. The solvent-carbon dioxide separator 9 separatesa solvent in a liquid state and carbon dioxide in a gas state. Inaddition, the solvent-carbon dioxide separator 9 is connected to thecompressor 2 downstream therefrom.

The solvent pumping apparatus 10 is provided on a flow path of thesolvent discharged from the solvent-carbon dioxide separator 9.

An operation of the cooling system 1A according to the above-mentionedembodiment will be described.

When the compressor 2 is driven, the carbon dioxide is pressurized toabout 10 MPaG to become the pressurized carbon dioxide. The pressurizedcarbon dioxide passes through the cooler 3 and then is cooled by thecooling water to reach about 10 MPaG and about 40° C.

Then, the cooled pressurized carbon dioxide is introduced to thepressurized carbon dioxide flow path 5 f, and passes through the heatexchange apparatus 5. Accordingly, in the vicinity of the outlet of thepressurized carbon dioxide flow path 5 f, the pressurized carbon dioxidereaches about 10 MPaG and about −45° C.

In addition, the solvent supplied from the solvent-carbon dioxideseparator 9 is introduced to the solvent flow path 5 e by the solventpumping apparatus 10. Accordingly, in the vicinity of the outlet ofsolvent flow path 5 e, the solvent reaches about −45° C.

The pressurized carbon dioxide and solvent passing through the heatexchange apparatus 5 are input into the mixer 6 and agitated.Accordingly, the pressurized carbon dioxide and solvent become a mixedrefrigerant in a liquid state. Then, the mixed refrigerant is suppliedto the depressurization apparatus 7. The mixed refrigerant isdepressurized from about 10 MPaG to about 0.5 MPaG to reach about −55°C. Some of the carbon dioxide in the mixed refrigerant is changed to agas state and separated through depressurization.

The depressurized mixed refrigerant is supplied to the mixed refrigerantflow path 5 c, and exchanges heat with the natural gas in the heatexchange apparatus 5. Accordingly, the mixed refrigerant reaches about35° C., and the carbon dioxide is vaporized. The mixed refrigerantpassing through the mixed refrigerant flow path 5 c is stored in thesolvent-carbon dioxide separator 9 and separated into carbon dioxide ina gas state and a solvent.

In addition, the carbon dioxide gas separated in the mixedrefrigerant-carbon dioxide separator 8 is supplied to the vaporizedcarbon dioxide flow path 5 d, and exchanges heat with the solvent andthe pressurized carbon dioxide in the heat exchange apparatus 5. Then,the carbon dioxide in a gas state passing through the vaporized carbondioxide flow path 5 d is stored in the solvent-carbon dioxide separator9, and returned to the compressor 2 again together with the carbondioxide in the mixed refrigerant separated in the solvent-carbon dioxideseparator 9.

In the above-mentioned cooling system 1A, the natural gas flow path 5 apasses through the natural gas flow path 5 a, and then is cooled fromabout 25° C. to about −50° C.

According to this embodiment, in the heat exchange apparatus 5, when thesecond heat exchanger (the vaporized carbon dioxide flow path 5 d, thesolvent flow path 5 e, and the pressurized carbon dioxide flow path 5 f)is provided, the pressurized carbon dioxide can be cooled by thevaporized carbon dioxide through depressurization. Therefore, energyefficiency can be improved. Furthermore, when the carbon dioxide and thesolvent are mixed, the problem of dry ice being formed in the apparatusdue to an operation mistake or the like can be prevented. In addition,the mixed refrigerant of a lower temperature than that in the relatedart can be provided by mixing the carbon dioxide and the solvent anddecompressing the mixed refrigerant to the pressure of a triple point orless, and cyclic cooling using the carbon dioxide in a temperature zoneof a triple point is also facilitated.

In addition, according to this embodiment, it is possible to cool thesolvent using the second heat exchanger. Accordingly, the solvent can becooled to the same temperature as the carbon dioxide before mixing withthe carbon dioxide, and a temperature of the mixed refrigerant can befurther decreased.

In addition, according to this embodiment, in the heat exchangeapparatus 5, the heat exchanger and the second heat exchanger areintegrated. Accordingly, energy efficiency can be increased withoutcomplicating the apparatus configuration of the cooling system 1A.

In addition, in the cooling system 1A, a valve is used for thedepressurization apparatus 7. Accordingly, a simple apparatusconfiguration that does not perform power recovery is realized.

Second Embodiment

A variant of the first embodiment will be described as a secondembodiment with reference to FIG. 2 . Further, the same components asthose of the first embodiment are designated by the same referencenumerals, and description thereof will be omitted.

A cooling system 1 according to this embodiment newly includes apre-cooler 4, and a power recovery turbine 7 a and a generator 7 b areprovided in the depressurization apparatus 7. Further, the heat exchangeapparatus 5 includes a nitrogen pre-cooling flow path 5 b.

The pre-cooler 4 is a heat exchanger provided downstream from the cooler3 and using carbon dioxide in a high pressure state as a primary sideand using carbon dioxide returned from the solvent-carbon dioxideseparator 9 as a secondary side. The pre-cooler 4 is configured to coolthe carbon dioxide in a high pressure state using the carbon dioxidereturned from the solvent-carbon dioxide separator 9. The carbon dioxidein a gas state passing through the vaporized carbon dioxide flow path 5d of the heat exchange apparatus 5 is stored in the solvent-carbondioxide separator 9, supplied to a secondary side of the pre-cooler 4together with the carbon dioxide in the mixed refrigerant separated inthe solvent-carbon dioxide separator 9, and returned to the compressor 2again. Accordingly, the pressurized carbon dioxide supplied from thecompressor 2 can be cooled by the carbon dioxide on the side returningto the compressor 2.

Nitrogen in a normal temperature (about 25° C.) state used as therefrigerant in the cooling system (not shown) on the downstream side issupplied to the nitrogen pre-cooling flow path 5 b in the same directionas the natural gas. The supplied nitrogen passes through the nitrogenpre-cooling flow path 5 b and then is cooled to about −50° C. Thenitrogen refrigerant passes through the nitrogen pre-cooling flow path 5b and then is cooled from about 35° C. to about −50° C.

The depressurization apparatus 7 is an apparatus configured to performpower generation using the generator 7 b by rotating the power recoveryturbine 7 a using a flow of the mixed refrigerant from the mixer 6toward the mixed refrigerant-carbon dioxide separator. That is, thedepressurization apparatus 7 is an apparatus configured to collectkinetic energy of the mixed refrigerant as electrical energy.

A pump 10 a is an apparatus configured to pump the solvent from thesolvent-carbon dioxide separator 9 to the solvent flow path 5 e. A motor10 b is connected to the pump 10 a to operate the pump 10 a. Inaddition, the motor 10 b is driven by feeding power from thedepressurization apparatus 7.

In addition, according to this embodiment, power of the mixedrefrigerant is collected by the depressurization apparatus 7 to generateelectric power. Accordingly, kinetic energy of the mixed refrigerant canbe extracted, and energy efficiency can be increased.

In addition, according to this embodiment, the solvent pumping apparatus10 receives electric power from the depressurization apparatus 7.Accordingly, the solvent pumping apparatus 10 is operated by kineticenergy of the mixed refrigerant, and energy efficiency can be furtherincreased.

In addition, according to this embodiment, in the heat exchangeapparatus 5, pre-cooling can be performed to a region of a lowertemperature than that in the related art, and the nitrogen refrigerantused on a further downstream side can be cooled. Accordingly, when thecooling system 1 is used for pre-cooling, cooling efficiency of thecooling system on the downstream side can be improved, and the naturalgas can be more efficiently cooled as a whole.

Third Embodiment

A variant of the first embodiment will be described as a thirdembodiment with reference to FIG. 7 . Further, the same components asthose of the first embodiment are designated by the same referencenumerals, and description thereof will be omitted.

A cooling system 1B, according to this embodiment, further includesthree compressors: 2A, 2B and 2C; three coolers: 3A, 3B and 3C; a frontstage separator, 8A; a front stage heat exchange apparatus, 11; a rearstage heat exchange apparatus, 12; and a pre-cooling apparatus, 13. Inaddition, in this embodiment, instead of the heat exchange apparatus 5,the heat exchange apparatus 5C is provided.

The compressors 2A, 2B and 2C are connected to each other in series. Inaddition, the coolers 3A, 3B and 3C are provided on the outlet sides ofthe compressors 2A, 2B and 2C, respectively. Accordingly, carbon dioxidedischarged from the compressors 2A, 2B and 2C is cooled by the coolers3A, 3B and 3C.

The heat exchange apparatus 5C according to this embodiment includes anatural gas flow path 5 a, a first mixed refrigerant flow path 5 h, asecond mixed refrigerant flow path 5 i and a solvent flow path 5 j. Themixed refrigerant passing through a mixed refrigerant flow path 11 b(which will be described below) is supplied to the first mixedrefrigerant flow path 5 h. In addition, the mixed refrigerant passingthrough the mixed refrigerant flow path 11 b (which will be describedbelow) is supplied to the second mixed refrigerant flow path 5 i in astate in which the mixed refrigerant is depressurized by thedepressurization apparatus 7B (opening of a valve). That is, the mixedrefrigerant flowing through the second mixed refrigerant flow path 5 ihas a temperature that is lower than that of the mixed refrigerantflowing through the first mixed refrigerant flow path 5 h. In addition,the second mixed refrigerant flow path 5 i is connected to the firstsolvent-carbon dioxide separator 9 a on the downstream side. The solventin a liquid state separated in the second solvent-carbon dioxideseparator 9 b is supplied to the solvent flow path 5 j.

The front stage separator 8A is an apparatus provided between the frontstage heat exchange apparatus 11 and the heat exchange apparatus 5C andconfigured to separate gaseous carbon dioxide from the mixed refrigerantdischarged from the front stage heat exchange apparatus 11.

The front stage heat exchange apparatus 11 includes a natural gas flowpath 11 a and the mixed refrigerant flow path 11 b. The natural gas flowpath 11 a is connected to an upstream side from the natural gas flowpath 5 a included in the heat exchange apparatus 5C. The mixedrefrigerant generated in the mixer 6 is supplied to the mixedrefrigerant flow path 11 b. In addition, the mixed refrigerant flow path11 b is connected to the front stage separator 8A on the downstreamside.

The rear stage heat exchange apparatus 12 includes a natural gas flowpath 12 a and a mixed refrigerant flow path 12 b. The natural gaspassing through the natural gas flow path 5 a of the heat exchangeapparatus 5C is supplied to the natural gas flow path 12 a. The mixedrefrigerant passing through the first mixed refrigerant flow path 5 h ofthe heat exchange apparatus 5C is supplied to the mixed refrigerant flowpath 12 b after being depressurized in the depressurization apparatus7C. The mixed refrigerant flow path 12 b is connected to the secondsolvent-carbon dioxide separator 9 b on the downstream side.

The pre-cooling apparatus 13 is an apparatus configured to previouslycool the pressurized carbon dioxide discharged from the compressor 2Cusing the carbon dioxide that is heat-exchanged in the heat exchangeapparatus 5C and the rear stage heat exchange apparatus 12. Thepre-cooling apparatus 13 includes: a pressurized carbon dioxide flowpath, 13 a; a first carbon dioxide flow path, 13 b; and a second carbondioxide flow path, 13 c. The pressurized carbon dioxide discharged fromthe compressor 2C is supplied to the pressurized carbon dioxide flowpath 13 a. The carbon dioxide passing through the second mixedrefrigerant flow path 5 i and separated in the first solvent-carbondioxide separator 9 a is supplied to the first carbon dioxide flow path13 b. In addition, the first carbon dioxide flow path 13 b is connectedto an inlet of the compressor 2B downstream therefrom. The carbondioxide passing through the mixed refrigerant flow path 12 b andseparated in the second solvent-carbon dioxide separator 9 b is suppliedto the second carbon dioxide flow path 13 c. In addition, the secondcarbon dioxide flow path 13 c is connected to the inlet of thecompressor 2A downstream therefrom.

In the cooling system 1B of this embodiment, the pressurized carbondioxide discharged from the compressor 2C is previously cooled by thepre-cooling apparatus 13 and then depressurized when the valve of thedepressurization apparatus 7A is opened, and thus, the temperaturethereof is decreased. Then, the depressurized carbon dioxide is suppliedto the mixer 6 together with the solvent supplied from the first andsecond solvent-carbon dioxide separator 9 a and 9 b and the mixedrefrigerant supplied from the front stage separator 8A, and then mixed.The mixed refrigerant mixed in the mixer 6 is supplied to the mixedrefrigerant flow path 11 b of the front stage heat exchange apparatus 11and exchanges heat with the natural gas flowing through the natural gasflow path 11 a.

Then, the mixed refrigerant passing through the mixed refrigerant flowpath 11 b is separated into the gaseous carbon dioxide and the mixedrefrigerant in the front stage separator 8A. The separated gaseouscarbon dioxide is supplied to the compressor 2C. In addition, the mixedrefrigerant separated in the front stage separator 8A is bifurcated at adiverging point D and supplied to the first mixed refrigerant flow path5 h and the second mixed refrigerant flow path 5 i, and some of themixed refrigerant joins the carbon dioxide and the solvent using a pump10 c as described above In addition, the mixed refrigerant supplied tothe first mixed refrigerant flow path 5 h is cooled through heatexchange with the depressurized mixed refrigerant flowing through thesecond mixed refrigerant flow path 5 i.

In addition, the mixed refrigerant is depressurized by thedepressurization apparatus 7B in front of the second mixed refrigerantflow path 5 i, and the temperature thereof is further decreased. Themixed refrigerant flowing through the second mixed refrigerant flow path5 i exchanges heat with the mixed refrigerant flowing through the firstmixed refrigerant flow path 5 h, the natural gas flowing through thenatural gas flow path 5 a, and the solvent flowing through the solventflow path 5 j.

The mixed refrigerant passing through the first mixed refrigerant flowpath 5 h is supplied to the mixed refrigerant flow path 12 b of the rearstage heat exchange apparatus 12 after the temperature thereof isdecreased when depressurization is performed again in thedepressurization apparatus 7C. Then, the mixed refrigerant flowingthrough the mixed refrigerant flow path 12 b is supplied to the secondsolvent-carbon dioxide separator 9 b after heat exchange with thenatural gas flowing through the natural gas flow path 12 a.

In addition, the mixed refrigerant passing through the second mixedrefrigerant flow path 5 i is supplied to the first solvent-carbondioxide separator 9 a. The mixed refrigerant in the first solvent-carbondioxide separator 9 a is separated into the gaseous carbon dioxide andthe mixed refrigerant. The solvent separated in the first solvent-carbondioxide separator 9 a is delivered to the mixer 6 upstream from thefront stage heat exchange apparatus 11 through the pump 10 c. Inaddition, the gaseous carbon dioxide separated in the firstsolvent-carbon dioxide separator 9 a is supplied to the first carbondioxide flow path 13 b of the pre-cooling apparatus 13.

The mixed refrigerant passing through the mixed refrigerant flow path 12b and supplied to the second solvent-carbon dioxide separator 9 b isseparated into the gaseous carbon dioxide and the solvent. The separatedsolvent passes through the solvent flow path 5 j and is supplied to themixed refrigerant flow path 11 b again. In addition, some of the solventseparated in the second solvent-carbon dioxide separator 9 b isdepressurized in the depressurization apparatus 7C and then returned tothe mixed refrigerant flow path 12 b. The gaseous carbon dioxide issupplied to the second carbon dioxide flow path 13 c.

According to the above-mentioned embodiment, the pressurized carbondioxide is depressurized in stages and heat-exchanged at every stage.Accordingly, the solvent and the mixed refrigerant can be self-cooledwhile cooling the natural gas in stages, and energy efficiency can beimproved.

Hereinabove, appropriate embodiments of the disclosure have beendescribed with reference to the accompanying drawings, the disclosure isnot limited to the above-mentioned embodiments. Shapes, combinations, orthe like, of the components in the above-mentioned configurations aremerely examples, and various modifications may be made based on designrequirements or the like without departing from the spirit of thedisclosure.

In the embodiment, while the cooling system 1 or 1A is a coolingapparatus configured to previously cool a natural gas, the disclosure isnot limited thereto. The cooling system 1 or 1A may be used, forexample, as a refrigeration system applied in the food industry. In thiscase, the cooling system 1 or 1A has a configuration in which a fluid tobe cooled is conveyed into the heat exchange apparatus 5, withoutincluding the natural gas flow path 5 a. In addition, the cooling system1 or 1A of the disclosure may be a cooling system for cryogenicseparation of a gas.

In addition, in the above embodiments, while the heat exchange apparatus5 is the plate fin type heat exchanger, the disclosure is not limitedthereto. For example, the heat exchange apparatus 5 may be a spiral typeheat exchanger.

In addition, the solvent is not limited as long as the solvent is amaterial that becomes a liquid state in a temperature zone of about −70°C.

In addition, as shown in FIG. 3 , the heat exchange apparatus 5according to the first or second embodiments can also be divided into aheat exchanger 5A and a second heat exchanger 5B (a second heatexchanger). The heat exchanger 5A includes the natural gas flow path 5a, and the mixed refrigerant flow path 5 c 1 in which the mixedrefrigerant supplied from the mixed refrigerant-carbon dioxide separator8 is guided. In addition, the second heat exchanger 5B includes themixed refrigerant flow path 5 c 2 in which the mixed refrigerantsupplied from the mixed refrigerant-carbon dioxide separator 8 isguided, the solvent flow path 5 e, and the pressurized carbon dioxideflow path 5 f.

In addition, as shown in FIG. 4 , the heat exchange apparatus 5 mayinclude a second mixed refrigerant flow path 5 g without including thesolvent flow path 5 e and the pressurized carbon dioxide flow path 5 f.In this case, the mixer 6 mixes the carbon dioxide and the solvent thatare compressed until they become a supercritical state by the compressor2 at an upper stage of the heat exchange apparatus 5. Accordingly, themixed refrigerant containing the pressurized carbon dioxide is suppliedto the second mixed refrigerant flow path 5 g.

In addition, as shown in FIG. 5 , the mixed refrigerant containing thepressurized carbon dioxide can also be cooled and depressurized bymixing the pressurized carbon dioxide and the solvent cooled in the heatexchange apparatus 5 in the mixer 6 and returning the mixed refrigerantto the heat exchange apparatus 5 again. In this case, in the heatexchange apparatus 5, the mixed refrigerant can be cooled, and the mixedrefrigerant can reach a lower temperature during depressurization.

In addition, as shown in FIG. 6 , the cooling system 1 may increase adischarge pressure of the mixed refrigerant feeding apparatus 8 a, mayset a pressure in the solvent-carbon dioxide separator 9 to be higherthan a pressure in the mixed refrigerant-carbon dioxide separator 8, andmay return the carbon dioxide passing through the vaporized carbondioxide flow path 5 d to the compressor 2. Accordingly, the pressure inthe mixed refrigerant flow path 5 c can be increased to increase avaporization temperature of the carbon dioxide in the flow path.Accordingly, a temperature gradient of the mixed refrigerant flow path 5c can be appropriately set according to a temperature gradient of thefluid to be cooled, the solvent or the pressurized carbon dioxide. Inaddition, when the discharge pressure of the mixed refrigerant feedingapparatus 8 a is increased and the pressure in the solvent-carbondioxide separator 9 is set to be higher, since the pressure of thevaporized carbon dioxide separated in the solvent-carbon dioxideseparator 9 can be increased, the power consumption of compressor 2 canbe reduced.

In addition, in the second embodiment, while the motor 10 b is drivenusing electric power collected by the depressurization apparatus 7, thedisclosure is not limited thereto. The electric power collected in thedepressurization apparatus 7 may be supplied to an external apparatus.

In addition, a variant of the third embodiment is shown in FIG. 8 . Incooling system 1C of the variant, the front stage heat exchangeapparatus 11 may include a first mixed refrigerant flow path 11 c and asecond mixed refrigerant flow path 11 d, instead of the mixedrefrigerant flow path 11 b. The solvent separated in the firstsolvent-carbon dioxide separator 9 a, the solvent passing through thesolvent flow path 5 j included in the heat exchange apparatus 5C, andthe carbon dioxide passing through the pre-cooling apparatus 13 aresupplied to the first mixed refrigerant flow path 11 c and the secondmixed refrigerant flow path 11 d. In addition, the mixed refrigerantsupplied to the first mixed refrigerant flow path 11 c is notdepressurized and becomes a high pressure state. In addition, the mixedrefrigerant supplied to the second mixed refrigerant flow path 11 d isdepressurized on an upstream side and becomes a state in which thetemperature is lowered. The first mixed refrigerant flow path 11 c isconnected to the first mixed refrigerant flow path 5 h and the secondmixed refrigerant flow path 5 i downstream therefrom. In addition, thesecond mixed refrigerant flow path 11 d is connected to the front stageseparator 8A downstream therefrom.

In addition, some of the solvent separated in the second solvent-carbondioxide separator 9 b is supplied to the mixed refrigerant flow path 12b. Further, the mixed refrigerant supplied to the mixed refrigerant flowpath 12 b is depressurized by the depressurization apparatus 7E.

According to the above-mentioned configuration, the cooling system 1Ccan cool the mixed refrigerant flowing through the first mixedrefrigerant flow path 5 h and in a pressurized state using the mixedrefrigerant flowing through the second mixed refrigerant flow path 5 i.Then, when the mixed refrigerant passing through the first mixedrefrigerant flow path 5 h is depressurized, the temperature of the mixedrefrigerant can be decreased to a lower temperature.

In the third embodiment, while the front stage heat exchange apparatus11 and the rear stage heat exchange apparatus 12 are provided, thepresent invention is not limited thereto. For example, when the heatexchange apparatus 5 of the first embodiment is changed to the heatexchange apparatus 5C, the mixed refrigerant flowing through the firstmixed refrigerant flow path 5 h can be cooled using the depressurizedmixed refrigerant.

Industrial Applicability

The disclosure can be used in a cooling system.

The invention claimed is:
 1. A cooling system comprising: a compressorconfigured to pressurize carbon dioxide to form pressurized carbondioxide; a mixer configured to generate a mixed refrigerant in which thepressurized carbon dioxide and a solvent in a liquid state are mixed; adepressurization apparatus provided downstream from the mixer andconfigured to depressurize the mixed refrigerant, the depressurizationapparatus including a valve; a separator configured to separate carbondioxide in a gas state from the mixed refrigerant; a heat exchangerconfigured to exchange heat between the mixed refrigerant cooled throughdepressurization and a fluid to be cooled; and a second heat exchangerconfigured to cool the pressurized carbon dioxide or the mixedrefrigerant using vaporized carbon dioxide or the mixed refrigerant,wherein the second heat exchanger is integrated with the heat exchanger,the heat exchanger is configured such that the mixed refrigerant and thefluid to be cooled always flow in opposite directions with respect toeach other in the heat exchanger, and the second heat exchanger isconfigured such that the vaporized carbon dioxide or the mixedrefrigerant and the pressurized carbon dioxide or the mixed refrigerantalways flow in opposite directions with respect to each other in thesecond heat exchanger.
 2. The cooling system according to claim 1,wherein the depressurization apparatus comprises a power recoveryturbine, and a power recovery apparatus configured to collect kineticenergy of the mixed refrigerant from the power recovery turbine.
 3. Thecooling system according to claim 1, wherein the second heat exchangeris provided upstream from the depressurization apparatus and isconfigured to cool the solvent using the vaporized carbon dioxide or thedepressurized mixed refrigerant.
 4. The cooling system according toclaim 1, wherein the second heat exchanger is provided downstream fromthe depressurization apparatus and configured to cool the mixedrefrigerant branched off upstream from the depressurization apparatususing the mixed refrigerant depressurized in the depressurizationapparatus.
 5. The cooling system according to claim 1, wherein thecooling system is configured to be depressurize the mixed refrigerant ina plurality of steps, and a plurality of the heat exchangers and aplurality of the second heat exchangers are provided and connected toeach other in series.